Prior to the middle of the last decade, superconductivity was restricted to such extremely low temperatures that it remained, primarily, a scientific curiosity with little practical value. The discovery of superconductivity in copper oxide-based ternary and quaternary systems at relatively high temperatures has generated extreme interest in the prospect that superconductors might, at last, find a niche in commercial technology. There are, however, practical difficulties.
One of the most promising systems for high temperature superconductors is based on the formula REBa.sub.2 Cu.sub.3 O.sub.7-x, where RE is a rare earth metal having an atomic number from 57 to 71, preferably yttrium, and x is a small number, less than 1. Murphy et al., Science, vol. 241 (1988) pp. 922-941, have provided an excellent review of the available processing techniques for the manufacture of YBa.sub.2 Cu.sub.3 O.sub.7-x high temperature superconductors. The Murphy et al. review indicates a number of difficulties in the processing techniques, including i) difficulties in achieving chemical homogeneity, ii) extreme brittleness and poor formability of the end and intermediate products, iii) "weak link" behavior arising from the tunneling of superconducting currents between isolated regions of superconductivity separated by thin non-superconductivity barriers, iv) the necessity for final oxygenation of the end product at 400.degree.-500.degree. C. and v) the presence of residual carbon.
For the synthesis of bulk YBa.sub.2 Cu.sub.3 O.sub.7-x superconductors, a broad spectrum of other processing techniques have been used, including (a) sol-gel processing (U.S. Pat. No. 4,977,109 of Liu and Barboux et al. Journal of Applied Physics, vol. 63, 1988, pp. 2725-29) (b) co-precipitation (U.S. Pat. Nos. 4,956,340 and 4,904,638 of Dicarolis and Barboux et al.supra, (c) use of nitrate or halide precursors (U.S. Pat. No. 4,861,753 of McCarron III and Japanese Patent 63256519), and (d) freeze drying (S. M. Johnson et al., Advanced Ceramic Materials, vol. 2, 1987,(pp 337-42).
Other processes, based on processing of appropriate intermediate precursors and aimed at avoiding submicron inhomogeneities include (i) chemical oxidation of liquid or solid metallic alloys for EuBa.sub.2 Cu.sub.3 O.sub.7 superconductors (U.S. Pat. No. 4,826,808 of Yurek et al.) (ii) oxidation of mechanically alloyed YBa.sub.2 Cu.sub.3 O.sub.7 powders (U.S. Pat. No. 4,962,085 of De Barbadillo II et al.) (iii) reaction of Y.sub.2 Cu.sub.2 O.sub.5 and BaCuO.sub.2 (U.S. Pat. No. 5,032,570 of Ogata et al.) (iv) reaction of Y.sub.2 BaCuO.sub.2, BaCuO.sub.2 and CuO (H. D. Kim et al., Jpn. J. Appl. Phys., vol. 29 (1990) pp. 2711-14), (v) reaction of Y.sub.2 Cu.sub.2 O.sub.5 and Ba.sub.3 Cu.sub.5 O.sub.5 (Tachikawa et al., Jpn. J. Appl. Phys., vol. 27, 1990, pp. L1501-1503), reaction of Y.sub.2 O.sub.3, BaO.sub.2 and CuO (U.S. Pat. No. 5,036,043 of Subramanian).
An objective of the present invention is to provide an effective and simplified process for making high temperature superconductors of the copper oxide family whereby there is significant improvement over the prior art in (i) chemical homogeneity achieved at better than micron level, (ii) impact brittleness problems at various processing stages are eliminated or minimized, (iii) carbon content of the superconductor is less than 0.10 weight percent, (iv) net shape processing can be achieved and (v) oxygenation of the end product is generally eliminated.
Sleight, Science, vol. 242 (1988), pp. 1519-1527, discloses that YBa.sub.2 Cu.sub.3 O.sub.7 is not stable at any condition of pressure and temperature; and that therefore "the synthesis of YBa.sub.2 Cu.sub.3 O.sub.7 requires two steps: first, YBa.sub.2 Cu.sub.3 O.sub.6+x is formed where x is approximately 0.5-0.8; and second, oxidation to YBa.sub.2 Cu.sub.3 O.sub.7 is carried out at a lower temperature."
In accordance with the invention described below YBa.sub.2 Cu.sub.3 O.sub.7 be synthesized in a single sintering step from a precursor; and there is no necessity for a separate post-sintering oxidation step, as indicated in the Sleight article.
Sleight, reveals with respect to Y-Ba-Cu-O systems that Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.6 is not a superconductor, but an antiferromagnetic insulator which may be indicated with respect to oxidation states as Y.sup.III Ba.sub.2.sup.II Cu.sub.2.sup.I Cu.sup.I O.sub.6 where I is monovalent, II is divalent and III is trivalent. It contains no trivalent copper. As the oxygen content is increased above the 6-level, Cu.sup.III atoms appear in the system along with superconductivity at higher temperatures until an oxygen level of close to 7 is obtained. Above this level, or above YBa.sub.2 Cu.sub.3 O.sub.7, additional oxygen results in the production of non-superconducting phase(s). In other words, superconductivity in such systems appears to require that some of the copper in the system be in the trivalent state, but not more than one of the three copper atoms in the formula can be trivalent if high temperature superconductivity is to be maintained.